Abstract
Cement is the ubiquitous material upon which modern civilisation is built, providing long-term strength, impermeability and durability for housing and infrastructure. The fundamental chemical interactions which control the structure and performance of cements have been the subject of intense research for decades, but the complex, crystallographically disordered nature of the key phases which form in hardened cements has raised difficulty in obtaining detailed information about local structure, reaction mechanisms and kinetics. Solid-state nuclear magnetic resonance (SS NMR) spectroscopy can resolve key atomic structural details within these materials and has emerged as a crucial tool in characterising cement structure and properties. This review provides a comprehensive overview of the application of multinuclear SS NMR spectroscopy to understand composition–structure–property relationships in cements. This includes anhydrous and hydrated phases in Portland cement, calcium aluminate cements, calcium sulfoaluminate cements, magnesia-based cements, alkali-activated and geopolymer cements and synthetic model systems. Advanced and multidimensional experiments probe 1H, 13C, 17O, 19F, 23Na, 25Mg, 27Al, 29Si, 31P, 33S, 35Cl, 39K and 43Ca nuclei, to study atomic structure, phase evolution, nanostructural development, reaction mechanisms and kinetics. Thus, the mechanisms controlling the physical properties of cements can now be resolved and understood at an unprecedented and essential level of detail.
Highlights
Modern society is heavily reliant on cementitious materials in constructing our built environment
Resonance at approximately diso 1⁄4 4e6 ppm, with this resonance shifting towards lower frequencies and exhibiting increased quadrupolar broadening with increasing silica content [50,362]. 27Al Magic angle spinning (MAS) NMR resonances attributed to katoite have been observed in calcium aluminate cements (CACs) [50,52] and alkali-activated slags [20,155,197]. 29Si MAS NMR analysis of hydrated CAC attributed a broad resonance at approximately diso 1⁄4 À79.9 ppm to Si sites in [Si(OAlVI)4-x] (OCa)x species and used spectral deconvolution to quantify this phase in the hydrated CAC [50]
This work has shown that the determination of alite and belite content in Portland cement (PC) using traditional Bogue calculations [379] strongly overestimates the belite content at the expense of alite, consistent with results from X-ray diffraction (XRD) and other techniques; the modified Taylor-Bogue calculation [380] provides a much better match to the alite/belite ratio determined by 29Si MAS NMR spectral deconvolution [27]
Summary
Modern society is heavily reliant on cementitious materials in constructing our built environment. By blending PC with SCMs, it is possible to enhance and control physical properties, e.g. strength, durability, phase formation and reaction kinetics, and to improve sustainability by reducing associated CO2 emissions and valorising industrial wastes [7]. NMR can be applied as a powerful tool probing the dynamics of mobile ions and water within cementitious materials, in particular through the use of 1H NMR relaxometry which provides information about water mobility and pore size distribution [8,9]. These various applications and nuclei will be discussed in detail in the sections which follow
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